Memory device

ABSTRACT

A memory device is provided, the memory device includes multiple cells arranged in a matrix of multiple rows and multiple columns. The memory device further includes multiple bit lines each of which is connected to first cells of the multiple cells arranged in a row of the multiple rows. A voltage control circuit is connectable to a selected bit line of the multiple bit lines and includes a voltage detection circuit that detects an instantaneous supply voltage and a voltage source selection circuit connected to the voltage detection circuit. The voltage source selection circuit selects a voltage source from multiple voltage sources based on the detected instantaneous supply voltage. The voltage source selection circuit includes a switch that connects the selected voltage source to the selected bit line to provide a write voltage.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/935,830, filed Nov. 15, 2019, the disclosure of which is herebyincorporated by reference in its entirety.

BACKGROUND

Integrated circuit (IC) memory devices include resistive memory, such asresistive random-access memory (RRAM), magnetoresistive random-accessmemory (MRAM), phase-change random-access memory (PCRAM), etc. Theresistive memory stores information by changing a resistance of adielectric material. For example, RRAM is a memory structure includingan array of RRAM cells each of which stores a bit of data usingresistance values, rather than electronic charge. Particularly, eachRRAM cell includes a resistive material layer, the resistance of whichcan be adjusted to represent logic “0” or logic “1”.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion. In addition, the drawings are illustrative as examples ofembodiments of the invention and are not intended to be limiting.

FIG. 1A is a block diagram generally illustrating an example memorydevice in accordance with some embodiments.

FIG. 1B is a block diagram generally illustrating an example cell arrayof a memory device in accordance with some embodiments.

FIG. 2A is a block diagram generally illustrating an example writevoltage circuit for a memory device in accordance with some embodiments.

FIG. 2B is a block diagram generally illustrating another example writevoltage circuit for a memory device in accordance with some embodiments.

FIG. 3 is a block diagram generally illustrating an example voltagecontrol circuit in accordance with some embodiments.

FIG. 4 is a block diagram generally illustrating an example temperaturecompensation circuit in accordance with some embodiments.

FIG. 5 is an example of a circuit for a PTAT current source inaccordance with some embodiments.

FIG. 6 is a block diagram illustrating a memory device with writevoltage circuit in accordance with example embodiments.

FIG. 7 is a block diagram illustrating placement of components in anexample memory device in accordance with example embodiments.

FIG. 8 is a flowchart of a method for providing a write voltage to amemory device in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

In some Integrated circuit (IC) memory devices, such as resistiverandom-access memory (RRAM), variations in bit-line (BL) or source line(SL) voltages during read and write operations occurs. In addition, thevariations in the BL voltage can also occur as a function of temperatureas well. This disclosure provides techniques for providing a suitablebit line voltage for write operations for a memory device and a way tocompensate for the variations in the bit line voltage during the writeoperations due to variations in temperature.

FIG. 1A is a block diagram illustrating an example memory device 100 inaccordance with some embodiments. In some examples, memory device 100 isa resistive memory device, such as resistive random-access memory(RRAM). As shown in FIG. 1A, memory device 100 includes a cell array102, a word line driver 104, an Input/Output (I/O) circuit 106, and awrite voltage circuit 108. Although write voltage circuit 108 is shownto be separate from I/O circuit 106, write voltage circuit 108 can bepart of I/O circuit 106. In addition, it will be apparent to a personwith ordinary skill in the art after reading this disclosure that memorydevice 100 can include more components or fewer components than thoseshown in FIG. 1.

FIG. 1B is a block diagram illustrating an example cell array 102 ofexample memory device 100 in accordance with some embodiments. As shownin FIG. 1B, cell array 102 includes a plurality of cells labeled as 110a 0, 110 m 0, 110 a 1, 110 m 1, 110 an, 110 mn (collectively referred toas plurality of memory cells 110). Each of the plurality of cells 110can store one bit of information (that is, a bit value 0 or a bit value1). Therefore, each of the plurality of cells 110 are also referred toas a bit cell or a memory cell.

In some examples, plurality of cells 110 of cell array 102 can includeresistive memory cells. Resistive memory cells include a resistiveelement having a layer of high-k dielectric material arranged betweenconductive electrodes disposed within a back-end-of-the-line (BEOL)metallization stack. Resistive memory cells are configured to operatebased upon a process of reversible switching between resistive states.This reversible switching is enabled by selectively forming a conductivefilament through the layer of high-k dielectric material. For example,the layer of high-k dielectric material, which is normally insulating,can be made to conduct by applying a voltage across the conductiveelectrodes to form a conductive filament extending through the layer ofhigh-k dielectric material. A resistive memory cell having a first(e.g., high) resistive state corresponds to a first data value (e.g., alogical ‘0’) and a resistive memory cell having a second (e.g., low)resistive state corresponds to a second data value (e.g., a logical‘1’).

As shown in FIG. 1B, the plurality of cells 110 of cell array 102 arearranged in a matrix having a plurality of rows (for examples, n rows)and a plurality of columns (for example, m columns). Each row of theplurality of rows includes a first plurality of cells of the pluralityof cells. For example, 0th row of cell array 102 include a firstplurality of cells labeled as 110 a 0, 110 m 0. Similarly, 1^(st) row ofcell array 102 includes a first plurality of cells labeled as 110 al,110 ml. Lastly, n^(th) row of cell array 102 includes a first pluralityof cells labeled as 110 an, . . . , 110 mn.

Similarly, each column of the plurality of columns includes a secondplurality of cells of the plurality of cells. For example, 0th column ofcell array 102 include a second plurality of cells labeled as 110 a 0,110 al, . . . , 110 an. Similarly, m^(th) column of cell array 102include a second plurality of cells labeled as 110 m 0, 110 m 1, . . . ,110 mn.

Cell array 102 further includes a plurality of word lines (for example,WL0, WL1, . . . , WLn) and a plurality of bit lines (for example, BL0, .. . , BLm). Each of the plurality of word lines is associated with a rowof the plurality of rows. For example, each of the first plurality ofcells in a row of the plurality of rows is connected to a word line ofthe plurality of word lines. As shown in FIG. 1B, the first plurality ofcells labeled as 110 a 0, . . . , 110 m 0 of the 0^(th) row areconnected to word line WL0. Similarly, the first plurality of cellslabeled as 110 a 1, . . . , 110 m 1 of the 1^(st) row are connected toword line WL1. Lastly, the first plurality of cells labeled as 110 an, .. . , 110 mn of the n^(th) row are connected to word line WLn.

Similarly, each bit line of the plurality of bit lines is associatedwith a column of the plurality of columns. For examples, each of thesecond plurality of cells of a column of the plurality of columns iscoupled to a bit line of the plurality of bit lines. As shown in FIG.1B, the second plurality of cells labeled as 110 a 0, 110 al, . . . ,110 an of the 0^(th) column are connected to bit line BL0. Similarly,the second plurality of cells labeled as 110 m 0, 110 m 1, . . . , 110mn of the m^(th) column are connected to bit line BLm.

Thus, each of the plurality of cells of cell array 102 is associatedwith an address defined by an intersection of a word line and a bitline. In some examples, cell array 102 further includes a plurality ofsource lines (for example, SL0, . . . , SLm). Each source line of theplurality of source line is also associated with a column of theplurality of columns. For example, the second plurality of cells of acolumn is coupled to a source line of the plurality of source lines. Asshown in FIG. 1B, the second plurality of cells labeled as 110 a 0, 110al, . . . , 110 an of the 0^(th) column are connected to source lineSL0. Similarly, the second plurality of cells labeled as 110 m 0, 110 m1, . . . , 110 mn of the m^(th) column are connected to source line SLm.

In some examples, and as shown in FIG. 1B, each of plurality of cells110 of cell array 102 include a resistive memory element 112 and anaccess transistor 114. Resistive memory element 112 has a resistivestate that is switchable between a low resistive state and a highresistive state. The resistive states are indicative of a data value(e.g., a “1” or “0”) stored within resistive memory element 112.Resistive memory element 112 has a first terminal coupled to a bit lineand a second terminal coupled to access transistor 114. Accesstransistor 114 has a gate coupled to a word line, a source coupled tosource line and a drain coupled to the second terminal of resistivememory element 112. In examples, access transistor 114 can besymmetrical. That is, a drain of access transistor 114 can be a sourceand a source of access transistor 114 can be a drain.

To read data from or to write data into cell array 102, a word line ofthe plurality of word lines is selected and charged to a predeterminedvoltage, for example, a word line voltage VWL. In addition, a bit lineof the plurality of bit lines and a source line of the plurality ofsource lines are selected and pre-charged to a predetermined voltages,for example, BL/SL voltages (VBL/VSL). The applied voltages cause asense amplifier to receive a signal having a value that is dependentupon a data state of a cell of cell array 102.

Returning to FIG. 1A, word line driver 104 selects a word line of theplurality of word lines and charge the selected word line to apredetermined voltage, for example, a word line voltage V_(WL). Wordline driver 104 selects the word line to charge based on decoding anaddress provided by a plurality of address lines. As shown in FIG. 1A,word line driver 104 is connected to cell array 102.

I/O circuit 106 applies the BL/SL voltages (that is, VBL/VSL) to aselected bit line and a selected source line during read-writeoperations. In some embodiments, I/O circuit 106 includes circuitry formultiplexing and encoding, and demultiplexing and decoding data to bewritten to, or read from, cell array 102, as well as circuitry forpre-charging a selected bit line and a selected source line forread-write operations. In some embodiments, I/O circuit 106 includescircuitry for amplifying read-write signals received from or applied toa selected bit line and a selected source line. In general, I/O circuit106 includes one or more circuitry necessary to control a selected bitline and a selected source line voltages for all SET, RESET, and READoperations executed on cell array 102 of resistive memory cells. Asshown in FIG. 1A, I/O circuit 106 is connected to cell array 102.

Continuing with FIG. 1A, write voltage circuit 108 provides a writevoltage to be applied to a selected bit line of cell array 102. Inaddition, write voltage circuit 108 compensates for variations in thewrite voltage due to variation in a temperature of cell array 102. Writevoltage circuit 108 improves write margins for cell array 102. Forexample, write voltage circuit 108 reduces variation in the writevoltage along the bit lines of cell array 102. In example embodiments,and as discussed in detail in the following sections of the disclosure,write voltage circuit 108 includes a voltage control circuit, alsoreferred to as a power switching system, which automatically selects asuitable power source for a write driver. Moreover, and as discussed indetail in the following sections of the disclosure, write voltagecircuit 108 also provides a temperature related reference signal toaccommodate mobility degradation on a write operation for cell array 102due to variation in a temperature of cell array 102.

FIG. 2A illustrates a block diagram illustrating write voltage circuit108 in accordance with example embodiments. As shown in FIG. 2A, writevoltage circuit 108 includes a voltage control circuit 200. Voltagecontrol circuit 200 (also referred to as a power switching system or apower switch scheme) uses a resistor ladder to detect a voltage level ofa power supply and compare it with a known voltage source (that is,VBG). Voltage control circuit 200 then uses the detected voltage levelto select a suitable power source for a write driver. The selection ofthe suitable power source is automatic. Voltage control circuit 200 isdescribed in greater detail with reference to FIG. 3 of the disclosure.

In addition, and as shown in FIG. 2B, in some examples, write voltagecircuit 108 further includes a temperature compensation circuit 210.Temperature compensation circuit 210 (also referred to as temperaturecompensation scheme) generates a temperature dependent reference signalfor the write driver. The temperature dependent reference signal is thenbe used to compensate the write-ability loss on high temperature andprevent device stress on low temperature. The temperature dependentreference signal is designed to align its level on a room temperature.No extra trimming may be needed. In addition, the temperature dependentreference signal is capable of adapting to different write voltagelevels automatically. Temperature compensation circuit 210 is describedin greater detail with reference to FIG. 4 of the disclosure.

FIG. 3 is a block diagram illustrating an example voltage controlcircuit 200 in accordance with some embodiments. As shown in FIG. 3,voltage control circuit 200 includes a voltage source selection circuit302 and a voltage detection circuit 304. Voltage source selectioncircuit 302 is connected to voltage detection circuit 304. Voltagedetection circuit 304 detects an instantaneous supply voltage (alsoreferred to as VDIO) and provides the detected instantaneous supplyvoltage to voltage source selection circuit 302. Voltage sourceselection circuit 302 selects a voltage source from a plurality ofvoltage sources based on the detected instantaneous supply voltage. Forexample, and as discussed in the following portions of the disclosure,voltage source selection circuit 302 includes a switch that connects theselected voltage source to a selected bit line to provide a writevoltage.

As shown in FIG. 3, voltage source selection circuit 302 includes aplurality of voltage sources 306 which includes, for example, a firstvoltage source 306 a and a second voltage source 306 b. In someexamples, first voltage source 306 a corresponds to a supply voltagelevel (that is, VDIO) and second voltage source 306 b corresponds to anincreased supply voltage level. For example, and as shown in FIG. 3,second voltage source 306 b includes a low ripple charge pump (that isLR-CP) 314 connected to a supply voltage node. LR-CP 314 increases thesupply voltage level thereby providing an increased supply voltage levelof second voltage source 306 b. Although voltage source selectioncircuit 302 is shown to include only two voltage sources (that is, firstvoltage source 306 a and a second voltage source 306 b), it will beapparent to a person with ordinary skill in the art after reading thisdisclosure that voltage source selection circuit 302 can include morethan two voltage sources.

In addition, voltage source selection circuit 302 includes a switch 308.In examples, switch 308 is a multi-domain power switch. For example,switch 308 is dual domain power switch. Switch 308 includes an inputterminal 310 and an output terminal 312. Switch 308 selects one ofplurality of voltage sources 306 and provide a voltage level associatedwith the selected one of plurality of voltage sources 306 at outputterminal 312. For instance, to select one of plurality of voltagesources 306, input terminal 310 of switch 308 is connected to a selectedvoltage source node. Switch 308 selects one of plurality of voltagesources 306 based on the instantaneous supply voltage. For example,switch 308 receives a signal representative of the instantaneous supplyvoltage from voltage detection circuit 304.

Voltage detection circuit 304 includes a resistor ladder 316 and a firstcomparator 318. Resistor ladder 316 includes a first resistor 330 and asecond resistor 332. A first terminal of first resistor 330 is connectedto a supply voltage node (that is, VDIO) and a second terminal of firstresistor 330 is connected to a first reference node 334. A first end ofsecond resistor 332 is connected to first reference node 334 and asecond terminal of second resistor 332 is connected to ground. Inexamples, a resistance value of first resistor 330 is equal to aresistance value of second resistor 332. However, it will be apparent toa person with ordinary skill in the art after reading this disclosurethat the resistance values of first resistor 330 and second resistor 332can be different. In addition, although resistor ladder 316 is shown toinclude only two resistors (that is, first resistor 330 and a secondresistor 332), it will be apparent to a person with ordinary skill inthe art after reading this disclosure that resistor ladder 316 caninclude more than two resistors.

Resistor ladder 316 provides a voltage which is a representative ofinstantaneous value of the supply voltage (that is, VDIO) at firstreference node 334. For example, first reference node 334 provides ahalf of the supply voltage (that is, ½(VDIO)). The representativevoltage of the instantaneous value of the supply voltage (also referredto as instantaneous supply voltage hereinafter) is provided to firstcomparator 318. First comparator 318 compares the instantaneous supplyvoltage with a reference voltage, for example, a band gate voltage (thatis, VBG). In some examples, first comparator 318 is an amplifier, suchas, an operational amplifier.

For example, first comparator 318 includes a first input terminal 320, asecond input terminal 322, and an output terminal 324. First inputterminal 320 of first comparator 318 is connected to a band gap voltagenode and second input terminal 322 of first comparator 318 is connectedto first reference node 334 of resistor ladder 316. First comparator 318compares the instantaneous supply voltage received at second inputterminal 322 with the reference voltage received at first input terminal320 and provides comparison result at output terminal 324. In exampleembodiments, the comparison result indicates whether the instantaneoussupply voltage is greater than or less than the reference voltage.

The comparison result from first comparator 318 is provided to switch308. For example, output terminal 324 of first comparator 318 isconnected to switch 308. Switch 308 selects one of plurality of voltagesources 306 based on the comparison result. For example, switch 308selects first voltage source 306 a of plurality of voltages sources 306when the comparison result indicates that the instantaneous supplyvoltage is equal to or more than the reference voltage. Moreover, switch308 selects second voltage source 306 b of plurality of voltage sources306 when the comparison result indicates that the instantaneous supplyvoltage is less than the reference voltage.

In some embodiments, voltage control circuit 200 includes a latch 326and a timer 328. Timer 328 keeps track of time periods and generates afirst trigger signal after a first predetermined time period andgenerates a second trigger signal after a second predetermined timeperiod. In some examples, the second trigger signal is generated afterthe first trigger signal. The first trigger signal triggers firstcomparator 318 to compare the instantaneous supply voltage with thereference voltage. The second trigger signal triggers latch 326 to storethe comparison result from first comparator 318. After latching of thecomparison result, first comparator 318 can be switched off to savepower. In example embodiments, latch 326 can be a flip-flop. Moreover,latch 326 can be used to speed up detection delay.

FIG. 4 is a block diagram generally illustrating an example temperaturecompensation circuit 210 in accordance with some embodiments. As shownin FIG. 4, temperature compensation circuit 210 includes a referencevoltage generator circuit 402 and a voltage regulator circuit 404.Voltage regulator circuit 404 is connected to reference voltagegenerator circuit 402. Reference voltage generator circuit 402 generatesa temperature compensated reference voltage and provides the temperaturecompensated reference voltage to voltage regulator circuit 404. Voltageregulator circuit 404 compares the temperature compensated referencevoltage with an instantaneous bit line voltage and regulates theinstantaneous bit line voltage based on the comparison.

As shown in FIG. 4, reference voltage generator circuit 402 includes afirst current source 406 and a second current source 408. First currentsource 406 is connected in parallel with second current source 408 at asecond reference node 410. First current source 406 sinks a firstcurrent at second reference node 410 and second current source 408 sinksa second current at second reference node 410. In some examples, firstcurrent source 406 is a Proportional to Absolute Temperature (PTAT)current source and the second current source 408 is a Zero TemperatureCoefficient (ZTC) current source. An example, PTAT current source isdiscussed in greater detail with reference to FIG. 5 of the disclosure.

In some examples, a PTAT current generated by the PTAT current source(that is, first current source 406) is proportional to an absolutetemperature and increases or decreases in a same direction as thetemperature increases or decreases. A ZTC current generated by the ZTCcurrent source (that is, second current source 408) have a temperaturecoefficient of zero with absolute temperature. That is, the ZTC currentis substantially invariable relative to absolute temperature. The PTATcurrent and the ZTC current are used in a combination to generate a biascurrent for reference voltage generator circuit 402. For example, slopes(that is rate of increase or decrease) of the PTAT current and the ZTCcurrent are controlled using trim codes so that the bias current forreference voltage generator circuit 402 remains same at a specifiedtemperature (for example, at room temperature) when the slopes arechanged.

Reference voltage generator circuit 402 further includes a variableresistor 412. A first terminal of variable resistor 412 is connected tosecond reference node 410 and a second terminal of variable resistor 412is connected to ground. In some examples, a resistance value of variableresistor 412 is changed to adjust the bias current for reference voltagegenerator 402. The temperature compensated reference voltage isgenerated at second reference node 410 and is provided at an outputterminal 414 of reference voltage generator circuit 402. In examples,the temperature compensated reference voltage generated at outputterminal 414 of reference voltage generator circuit 402 is provided tovoltage regulator circuit 404.

Continuing with FIG. 4, voltage regulator circuit 404 includes a secondcomparator 416 and a third current source 418. Third current source 418is connected to second comparator 416. Second comparator 416 includes afirst input terminal 420, a second input terminal 422, and an outputterminal 424. First input terminal 420 is connected to output terminal414 of reference voltage generator circuit 402. Second input terminal422 is connected to a selected bit line of cell array 102. In examples,second comparator 416 compares the temperature compensated referencevoltage received at first input terminal 420 with an instantaneous bitline voltage received at second input terminal 422 and provides acomparison result on output terminal 424. The comparison result mayinclude whether the instantaneous bit line voltage is less than orgreater than the temperature compensated reference voltage.

The comparison result is provided to third current source 418. Thirdcurrent source 418 varies an amount of a source current Is being sinkedto the selected bit line based on the comparison result. For example,third current source 418 increases the amount of the source current Isbeing sinked to the selected bit line when the temperature compensatedreference voltage is more than the instantaneous bit line voltage.Moreover, third current source 418 decreases the amount of the sourcecurrent Is being sinked to the selected bit line when the temperaturecompensated reference voltage is less than the instantaneous bit linevoltage.

In examples, third current source 418 comprises a transistor 426. Asource of transistor 426 is connected to a write voltage node and adrain of transistor 426 is connected to a selected bit line of cellarray 102. A gate of transistor 426 is connected to output terminal 424of comparator 416. In some examples, transistor 426 is a p-channel metaloxide semiconductor (pMOS) transistor. However, it will be apparent to aperson with an ordinary skill in the art after reading this disclosurethat other types of transistors, such as, a metal oxide semiconductorfield effect transistor (MOSFET), an n-channel metal oxide semiconductor(nMOS) transistor, or a complementary metal oxide semiconductor (CMOS)transistor can be used for transistor 426. In addition, transistor 426is symmetrical. That is, a source of transistor 426 can be a drain, anda drain of transistor 426 can be a source.

In examples, a unigain buffer can be connected between reference voltagegenerator circuit 402 and voltage regulator circuit 404. The unigainbuffer shields reference voltage generator circuit 402 from kick backnoise generated by voltage regulator circuit 404. In other examples,voltage regulator circuit 404 is a low dropout (LDO) circuit. In someexamples, second comparator 416 is an amplifier, such as, an operationalamplifier.

FIG. 5 is an example of a circuit of a PTAT current source 500 inaccordance with some embodiments. In example embodiments, PTAT currentsource 500 includes a bandgap reference (BGR) circuit. PTAT currentsource 500 is a temperature independent current source that outputs afixed (constant) current regardless of temperature changes. In someexamples, PTAT current source 500 outputs a PTAT current that varieslinearly with temperature. As shown in FIG. 5, PTAT current source 500includes a first transistor Q1 502 and second transistor Q2 504. Inaddition, PTAT current source 500 includes a first resistor R1 506, asecond resistor R2 508, a third resistor R3 510, and a fourth resistorR4 512. Moreover, PTAT current source 500 includes a third comparator514, a first current mirror 516, and a second current mirror 518.

First transistor Q1 502 is connected between a first node 520 andground. A first terminal of first resistor R1 506 is connected to athird node 524 and a second terminal of first resistor R1 506 isconnected to first node 520. A first terminal of second resistor R2 508is connected to third node 524 and a second terminal of second resistorR2 508 is connected to a second node 522. A first terminal of thirdresistor R3 510 is connected to second node 522 and a second terminal ofthird resistor R3 510 is connected to second transistor 504. Secondtransistor Q2 504 is connected between third resistor R3 510 and ground.In some examples, first transistor Q1 502 and second transistor Q2 504are bipolar junction transistors (BJTs). However, other types oftransistors are within the scope of the disclosure.

A first input terminal of third comparator 514 is connected to firstnode 520 and a second input terminal of third comparator 514 isconnected to second node 522. An output terminal of third comparator 514is connected to a fourth node 526. First current mirror 516 is connectedto third node 524 and sinks a first matched current at third node 524.Second current mirror 518 is connected to a first terminal of fourthresistor R4 512 and sinks a second matched current into fourth resistorR4 512. A control gate for each of first current mirror 516 and secondcurrent mirror is connected to the output terminal of third comparator514 at fourth node 526. A second terminal of fourth resistor R4 512 isconnected to ground.

In examples, a voltage of first node 520 is represented as v1 and avoltage of second node 522 is represented by v2. Third comparator 514compares v1 with v2 and based on the comparison regulates the firstmatched current and the second matched currents being sinked by firstcurrent mirror 516 and second current mirror 518 respectively. Forexample, third comparator 514 regulates the first matched current andthe second matched currents being sinked by first current mirror 516 andsecond current mirror 518 respectively such that v1 is approximatelyequal to v2. In examples, first matched current being sinked by firstcurrent mirror 516 is approximately equal to the second matched currentbeing sinked by second current mirror 518.

In examples, a current flowing through first resistor R1 506 isrepresented as I1, a current flowing through second resistor R2 508 isrepresented as I2, and a current flowing through fourth resistor R4 512is represented as I3. In some examples, a resistance value of firstresistor R1 506, represented as R1, is approximately equal to aresistance value of second resistor R2 508 represented as R2. That is,R1=R2. In addition, since v1 is approximately equal to v2, I1 isapproximately equal to I2. Hence, using BJT equations:

$I_{2} = {\frac{V_{{BE}\; 1} - V_{{BE}\; 2}}{R_{3}} = {\frac{V_{T}\ln\frac{I_{C}}{I_{S}}V_{T}\ln\frac{I_{C}}{{nl}_{S}}}{R_{3}} = \frac{V_{T}\ln\; n}{R_{3}}}}$${IPTAT} = {I_{3} = {{KI}_{2} = \frac{{KV}_{T}\ln\; n}{R_{3}}}}$where V_(T) is linearly proportional to temperature and n is the ratioof emitter areas of transistors Q1 and Q2. I3 is proportional to I2applied to the gate of second current mirror 518 by a factor of K.Because V_(T) varies linearly with temperature, IPTAT (that is, the PTATcurrent) also varies linearly with temperature.

FIG. 6 is a block diagram illustrating a memory device 600 with writevoltage circuit 108 in accordance with example embodiments. As shown inFIG. 6, memory device 600 includes voltage source selection circuit 302and voltage detection circuit 304 connected to voltage source selectioncircuit 302. In addition, and as shown in FIG. 6, memory device 600further includes reference voltage generator circuit 402 and voltageregulator circuit 404 connected to reference voltage generator circuit402. Voltage source selection circuit 302 is connected to voltageregulator circuit 404. Moreover, voltage regulator circuit 404 isconnected to cell array 102.

Voltage detection circuit 304 detects an instantaneous supply voltage(also referred to as VDIO) and provides the detected instantaneoussupply voltage to voltage source selection circuit 302. Voltage sourceselection circuit 302 selects a voltage source from plurality of voltagesources 306 based on the detected instantaneous supply voltage. Forexample, voltage source selection circuit 302 includes switch 308 thatconnects the selected voltage source to a selected bit line to provide awrite voltage (that is, Vo). Reference voltage generator circuit 402generates a temperature compensated reference voltage and provides thetemperature compensated reference voltage to voltage regulator circuit404. Voltage regulator circuit 404 compares the temperature compensatedreference voltage with an instantaneous bit line voltage (that is, VBL)and adjusts the instantaneous bit line voltage (that is, VBL) based onthe comparison. In example embodiments, voltage source selection circuit302, voltage detection circuit 304, and reference voltage generatorcircuit 402 can be shared with multiple cell arrays of memory device600.

FIG. 7 is a block diagram illustrating placement of components in anexample memory device 700 in accordance with example embodiments. Asshown in FIG. 7, a first cell array 102 a is placed in a first sectionof a cell area. The first section extends along a first edge 702 from athird edge 706 of the cell area to an opposite fourth edge 708. A secondcell array 102 b is placed in a second section of the cell area. Thesecond section extends along a second edge 704 from third edge 706 ofthe cell area to fourth edge 708. Second edge 704 is opposite first edge702.

A first voltage regulator circuit 404 a is placed in a third section ofthe cell area. The third section is adjacent to the first section. Thethird section extends along the first section from third edge 706 tofourth edge 708. A second voltage regulator circuit 404 b is placed in afourth section of the cell area. The fourth section is adjacent to thesecond section. The fourth section extends along the second section fromthird edge 706 to fourth edge 708. LR-CP 314, voltage control circuit200, and temperature compensation circuit 210 is placed in a fifthsection of the cell area. The fifth section is sandwiched between thethird section and the fourth section and extends from edge 706 to fourthedge 708.

For example, LR-CP 314 is placed in a first sub-section of the fifthsection. Voltage control circuit 200 is placed in a second sub-sectionof the fifth section. The second sub-section is next to or proximate tothe first sub-section. Temperature compensation circuit 210 is placed ina third sub-section of the fifth section. The third sub-section is nextto or proximate to the second sub-section. The second sub-section issandwiched between the first sub-section and the third sub-section.Thus, voltage control circuit 200 is placed next to or in proximity toLR-CP 314. In addition, temperature compensation circuit 210 is placednext to or in proximity to voltage control circuit 200. However, otherplacements are within the scope of the disclosure.

FIG. 8 is a flowchart of a method 800 for providing a write voltage to amemory device in accordance with some embodiments. The method 800 can beperformed, for example, by write voltage circuit 108 as discussed withreference to any of FIGS. 1A-7. In some embodiments, method 800 can bestored as instructions on a non-transitory computer readable mediumwhich can be executed by a processor to perform method 800.

At block 810 of method 800, an instantaneous supply voltage is detected.For example, resistor ladder 316 of voltage detection circuit 304detects an instantaneous supply voltage. A voltage signal representativeof the instantaneous supply voltage is provided at reference node 334.

At block 820 of method 800, the instantaneous supply voltage is comparedwith a reference voltage. For example, first comparator 318 of voltagedetection circuit compares the instantaneous supply voltage with thereference voltage. The instantaneous supply voltage is provided atsecond input terminal 322 of first comparator 318 and the referencevoltage is provided at first input terminal 320 of first comparator 318.

At block 830 of method 800, a voltage source from a plurality of voltagesources is selected based on comparing the instantaneous supply voltagewith the reference voltage. For example, first comparator 318 providesan output signal having comparison results at output terminal 324. Theoutput signal, for example, may indicate whether the instantaneoussupply voltage is less than, equal to, or greater than the referencevoltage. The output signal having the comparison results is provided toswitch 308 of voltage source selection circuit 302. Switch 308 thenselects one of plurality of voltage sources 306 based on the comparisonresults. For example, switch 308 selects first voltage source 306 a whenthe instantaneous supply voltage is equal to or greater than thereference voltage. In other examples, switch 310 selects second voltagesource 306 b when the instantaneous supply voltage is less thanreference voltage.

At block 840 of method 800, the selected voltage source is connected toa selected bit line of a cell array to provide a write voltage to theselected bit line. For example, switch 308 of voltage source selectioncircuit 302 connects the selected voltage source to a selected bit lineof a cell array through switch input terminal 310 and switch outputterminal 312 to provide a write voltage to the selected bit line.

At block 850 of method 800, a temperature adjusted reference voltage isgenerated. For example, reference voltage generator circuit 402generates a temperature adjusted reference voltage. In exampleembodiments, the temperature adjusted reference voltage is generatedusing first current source 406 (that is, the PTAT current source) andsecond current source 408 (that is, the ZTC current source). Thetemperature adjusted reference voltage is provided at output terminal414 of reference voltage generator circuit 402.

At block 860 of method 800, an instantaneous write voltage is detected.At block 870 of method 800, the instantaneous write voltage is comparedwith the temperature adjusted reference voltage. In example embodiments,second comparator 416 of voltage regulator circuit 404 compares theinstantaneous write voltage with the temperature adjusted referencevoltage. The temperature adjusted reference voltage is provided at firstinput terminal 420 of second comparator 416 and the instantaneous writevoltage is provided at second input terminal 422 of second comparator416.

At block 880 of method 800, the instantaneous write voltage is regulatedbased on comparing the instantaneous write voltage with the temperatureadjusted reference voltage. For example, second comparator 416 providesan output signal having comparison results at output terminal 424. Theoutput signal, for example, may indicate whether the instantaneous writevoltage is less than, equal to, or greater than the temperature adjustedreference voltage. The output signal having the comparison results isprovided to a gate of third current source 418. Third current source 418then increases or decreases the source current Is being sinked to theselected bit line. For example, third current source 418 decreases thesource current Is being sinked to the selected bit line when theinstantaneous write voltage is equal to or greater than the temperatureadjusted reference voltage. In other examples, third current source 418increases the source current Is being sinked to the selected bit linewhen the instantaneous write voltage is less than the temperatureadjusted reference voltage.

Disclosed embodiments thus provide a memory device comprising: aplurality of cells arranged in a matrix comprising a plurality of rowsand a plurality of columns; a plurality of bit lines, wherein each ofthe plurality of bit lines is connected to a first plurality of cells ofthe plurality of cells arranged in a column of the plurality of columns;a voltage control circuit connectable to a selected bit line of theplurality of bit lines, wherein the voltage control circuit comprises: avoltage detection circuit, wherein the voltage detection circuit detectsan instantaneous supply voltage; and a voltage source selection circuitconnected to the voltage detection circuit, wherein the voltage sourceselection circuit selects a voltage source from a plurality of voltagesources based on the detected instantaneous supply voltage, and whereinthe voltage source selection circuit comprises a switch that connectsthe selected voltage source to the selected bit line to provide a writevoltage.

In accordance with other disclosed examples, a memory device comprises:a cell array comprising a plurality of cells; a plurality of bit lines,wherein each of the plurality of bit lines is connected to a firstplurality of cells of the plurality of cells arranged in a column of thecell array; a voltage control circuit connectable to a selected bit lineof the plurality of bit lines, wherein the voltage control circuitprovides a write voltage to a selected bit line of the plurality of bitlines for a write operation; and a temperature compensation circuitconnectable to the selected bit line, wherein the temperaturecompensation circuit comprises: a reference voltage generator circuit,wherein the reference voltage generator circuit generates a temperatureadjusted reference voltage, and a voltage regulator circuit connected tothe reference voltage generator circuit, wherein the voltage regulatorcircuit compares an instantaneous write voltage with the temperatureadjusted reference voltage and regulates the instantaneous write voltagebased on the comparison.

In accordance with still further disclosed examples, a method ofproviding a write voltage comprises: detecting an instantaneous supplyvoltage; comparing the instantaneous supply voltage with a referencevoltage; selecting a voltage source from a plurality of voltage sourcesbased on comparing of the instantaneous supply voltage with thereference voltage; and connecting the selected voltage source to aselected bit line of a cell array to provide a write voltage to theselected bit line. In example embodiments, the method further comprises:generating a temperature adjusted reference voltage; detecting aninstantaneous write voltage; comparing the instantaneous write voltagewith the temperature adjusted reference voltage; and regulating theinstantaneous write voltage based on comparing the instantaneous writevoltage with the temperature adjusted reference voltage.

This disclosure outlines various embodiments so that those skilled inthe art may better understand the aspects of the present disclosure.Those skilled in the art should appreciate that they may readily use thepresent disclosure as a basis for designing or modifying other processesand structures for carrying out the same purposes and/or achieving thesame advantages of the embodiments introduced herein. Those skilled inthe art should also realize that such equivalent constructions do notdepart from the spirit and scope of the present disclosure, and thatthey may make various changes, substitutions, and alterations hereinwithout departing from the spirit and scope of the present disclosure.

What is claimed is:
 1. A memory device, comprising: a plurality of cellsarranged in a matrix comprising a plurality of rows and a plurality ofcolumns; a plurality of bit lines, wherein each of the plurality of bitlines is connected to a first plurality of cells of the plurality ofcells arranged in a column of the plurality of columns; and a voltagecontrol circuit connectable to a selected bit line of the plurality ofbit lines, wherein the voltage control circuit comprises: a voltagedetection circuit, wherein the voltage detection circuit detects aninstantaneous supply voltage; and a voltage source selection circuitconnected to the voltage detection circuit, wherein the voltage sourceselection circuit selects a voltage source from a plurality of voltagesources based on the detected instantaneous supply voltage, and whereinthe voltage source selection circuit comprises a switch that connectsthe selected voltage source to the selected bit line to provide a writevoltage.
 2. The memory device of claim 1, wherein the voltage detectioncircuit comprises a resistor ladder connected to a supply voltage nodeand a comparator connected to the resistor ladder, wherein the resistorladder detects the instantaneous supply voltage, and wherein thecomparator compares the detected instantaneous supply voltage with areference voltage.
 3. The memory device of claim 2, wherein an outputterminal of the comparator is connected to the switch, and wherein theswitch selects the voltage source from the plurality of voltage sourcesbased on an output of the comparator.
 4. The memory device of claim 3,wherein the switch selects a first voltage source of the plurality ofvoltages sources when the instantaneous supply voltage is less than thereference voltage, and wherein the switch selects a second voltagesource of the plurality of voltage sources when the instantaneous supplyvoltage is more than the reference voltage.
 5. The memory device ofclaim 2, wherein the voltage control circuit further comprises a timer,wherein the timer tracks a time period and generates, after expirationof the time period, a comparator enable signal, wherein the comparatorenable signal triggers the comparator to compare the detectedinstantaneous supply voltage with the reference voltage.
 6. The memorydevice of claim 5, wherein the voltage control circuit further comprisesa latch circuit, wherein the latch circuit latches the output of thecomparator.
 7. The memory device of claim 6, wherein the latch circuitis triggered by a clock signal, wherein the clock signal is generated bythe timer.
 8. The memory device of claim 6, wherein the voltagedetection circuit is switched off after latching of the output.
 9. Thememory device of claim 2, wherein the resistor ladder comprises a firstresistor and a second resistor, wherein a first terminal of the firstresistor is connected to a first supply voltage node, wherein a secondterminal of the first resistor is connected to an output node, wherein afirst terminal of the second resistor is connected to the output node,wherein a second terminal of the second resistor is connected to ground,and wherein the output node provides the instantaneous supply voltage.10. The memory device of claim 9, wherein a first input terminal of thecomparator is connected to the output node, and wherein a second inputterminal of the comparator is connected to a reference voltage node. 11.The memory device of claim 1, further comprising a temperaturecompensation circuit connectable to the selected bit line, wherein thetemperature compensation circuit compensates the write voltage for avariation in a temperature of the memory device.
 12. A memory device,comprising: a cell array comprising a plurality of cells; a plurality ofbit lines, wherein each of the plurality of bit lines is connected to afirst plurality of cells of the plurality of cells arranged in a columnof the cell array; a voltage control circuit connectable to a selectedbit line of the plurality of bit lines, wherein the voltage controlcircuit provides a write voltage to the selected bit line of theplurality of bit lines for a write operation; and a temperaturecompensation circuit connectable to the selected bit line, wherein thetemperature compensation circuit comprises: a reference voltagegenerator circuit, wherein the reference voltage generator circuitgenerates a temperature adjusted reference voltage, and a voltageregulator circuit connected to the reference voltage generator circuit,wherein the voltage regulator circuit compares an instantaneous writevoltage with the temperature adjusted reference voltage and regulatesthe instantaneous write voltage based on the comparison.
 13. The memorydevice of claim 12, wherein the reference voltage generator circuitcomprises: a first current source; a second current source connected inparallel to the first current source at a reference node; and a variableresistor connected between the reference node and ground, wherein thereference node provides the temperature adjusted reference voltage. 14.The memory device of claim 13, wherein the first current source is aProportional to Absolute Temperature (PTAT) current source and thesecond current source is a Zero Temperature Coefficient (ZTC) currentsource.
 15. The memory device of claim 12, wherein the voltage regulatorcircuit comprises an amplifier and a third current source, wherein theamplifier comprises a first input terminal, a second input terminal, andan output terminal, wherein the first input terminal of the amplifier isconnected to a reference voltage node, wherein the second input terminalof the amplifier is connected to the selected bit line, and wherein theoutput terminal of the amplifier is connected to the third currentsource.
 16. The memory device of claim 15, wherein the amplifierregulates an amount of current sinked by the third current source to theselected bit line based on comparing the instantaneous write voltagewith the temperature compensated reference voltage.
 17. The memorydevice of claim 12, further comprising a unigain buffer connectedbetween the reference voltage generator circuit and the voltageregulator circuit.
 18. The memory device of claim 12, wherein thevoltage regulator circuit comprises a low dropout (LDO) circuit.
 19. Amethod of providing a write voltage, the method comprising: detecting aninstantaneous supply voltage; comparing the instantaneous supply voltagewith a reference voltage; selecting a voltage source from a plurality ofvoltage sources based on comparison of the instantaneous supply voltagewith the reference voltage; and connecting the selected voltage sourceto a selected bit line of a cell array to provide the write voltage tothe selected bit line.
 20. The method of claim 19, further comprising:generating a temperature adjusted reference voltage; detecting aninstantaneous write voltage; comparing the instantaneous write voltagewith the temperature adjusted reference voltage; and regulating theinstantaneous write voltage based on comparing the instantaneous writevoltage with the temperature adjusted reference voltage.